Bar code scanners and other optical devices use focused beams of light. For example, in one type of bar code scanner, a beam of light from a laser diode is projected along an optical axis and focused to a spot of light at a focal location in a working region along the axis. A moving mirror positioned in the path of the beam sweeps the spot along a line of motion within a working region. A photo detector detects light reflected from the working region and converts the reflected light to an electrical signal. If an object bearing a pattern of light and dark regions such as a typical bar code is placed in the working region so that the moving spot sweeps across these regions, the reflected light will vary in a pattern corresponding to the pattern of light and dark regions. The electrical signal will vary in the same manner. The electrical signal can be detected and decoded to yield the information stored in the bar code.
The size and shape of the spot of light will affect the scanning operation. Typically, the light which forms the spot has non-uniform intensity, which decreases gradually adjacent the periphery of the spot. The area in which the intensity exceeds a selected value, such as a selected proportion of the maximum intensity, is regarded as the spot. If the spot is too large in relation to the light and dark features to be detected, the light will illuminate both light and dark features simultaneously with substantial intensity, and hence it will be impossible to read the bar code properly. In many cases, a non-circular spot is desirable. Typically, the spot has a long dimension transverse to the direction of motion of the spot and a short dimension in the direction of motion. For example, this arrangement can be used in scanners which are intended to read a conventional one-dimensional bar code consisting of a pattern of light and dark strips extending parallel to one another.
In some bar code scanning applications, the distance between the scanner and the object bearing the code may vary from object to object. For these applications, it is important to provide a light beam with a substantial depth of field, i.e., a beam which has relatively small dimensions throughout a substantial range of locations around the nominal focal location. In other applications, it is more important to assure that the beam focuses to the minimum breadth at the nominal focal location, to provide the smallest beam “waist” and hence provide the smallest spot size at the nominal focal location.
Typical light source apparatus used heretofore has incorporated a laser diode, a collimating lens for focusing the light from the diode to a spot at a focal location, and an opaque plate with an aperture disposed between the lens and the focal spot for blocking light at the periphery of the beam. The characteristics of the beam depend upon the characteristics of the aperture. For example, a non-circular aperture may be used to form a non-circular beam. A large aperture yields a beam with a low “f-number” which is sharply focused to a small size at the nominal focal location but which increases in size rapidly with distance from the nominal focal location, i.e., a beam with good resolution but poor depth of field A narrow aperture yields a beam with a high f-number which has a somewhat larger spot size at the nominal focal location but which increases in size more slowly with distance from the nominal focal location. Such a beam has relatively poor resolution but good depth of field. Systems of this type are disclosed, for example, in U.S. Pat. Nos. 4,816,660 and 5,247,162.
There has been a need heretofore for further improvements in light beam sources and in scanning apparatus incorporating the same.